There is a large body of research dealing with the question of the influence of the mother tongue on a foreign language being learnt, but relatively little which addresses cross-linguistic influence between two foreign languages. This thesis deals with this issue, examining specifically Spanish and Portuguese. The findings suggest that such influence does indeed occur, but not across the board. This study involved two features, clitic pronouns, and verbs of existence, thus comparing a more syntactic type of structure with a more semantic type of structure. Two different types of task were used in the study, grammaticality judgements and controlled production tasks, to compare knowledge with actual performance. No significant differences were found between their scores on the two tasks. Nor were there significant differences between their performance on the two structures, except on one aspect of the production task. The study also involved three different levels of students, whose scores were compared to ascertain whether transfer decreased with proficiency; this appeared to be the case for the judgements but not for the production task. The subjects were also asked to complete a grammaticality judgement task in Spanish, to see whether cross-linguistic influence was bi-directional; this would appear to be the case. As well as linguistic factors, this study also explores affective variables, as previous research in this area examined the relationship between these and general proficiency, but did not look at the specific question of cross-linguistic influence. Accordingly, students' performance on the tasks was compared with their answers to questionnaires on attitude and motivation. Some positive correlations were found between attitudes to the target culture and the occurrence of transfer, but again not across the board. Motivation-type appeared to have an effect on performance in the production task, but not in the judgement task, such that integrative motivation seemed to be linked to transfer between the two foreign languages. Finally some proposals are offered for classroom application of this research, followed by suggestions for further research.